Display device and controlling method thereof

ABSTRACT

A conventional setting voltage was a value with an estimated margin of a characteristic change of a light, emitting element. Therefore, a voltage between the source and drain of a driver transistor V ds  had to be set high (V ds ≧V gs −V Th +a). This caused high heat generation and power consumption, because a voltage applied to the light emitting element. The invention is characterized by,feedbacking a change in a current value in accordance with the deterioration of a light emitting element and a power source voltage controller which modifies a setting voltage. Namely, according to the invention, the setting voltage is to be set in the vicinity of the boundary (critical part) between ,a saturation region and a linear region,. and a voltage margin for the. deterioration is not required particularly for an initial setting voltage.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a display device (light emittingdevice) having a light emitting element, and more particularly such adisplay device having a display part of current driving method.

[0003] 2. Description of the, Related Art

[0004] Digital gradation method (digital driving method) and analoggradation method (analog driving method) can be given as driving methodof multi-color expression on a display device having a light emittingelement. In the aforementioned digital gradation method, a lightemitting element is driven in binary of ON (brightness is almost100%),and OFF (brightness is almost 0%) to obtain gradation bycontrolling the luminous region and the length of the period duringwhich each pixel emits light. In the analog gradation method, analoginput. data is written into a light emitting element to modulate thegradation in an analog manner.

[0005] Furthermore, expression of gradation is made through two methods,which are a constant voltage drive which is dependent on the voltageapplied to the light emitting element, and a constant current drivewhich is dependent on the current applied to the light emitting element.An electric current flowing through a light emitting element iscontrolled by a transistor (hereinafter referred to as drivertransistor) in current drive.

[0006] Operation of a driver transistor is explained referring to theV-I feature shown in FIG. 8. There are two operating regions of thedriver transistor, namely, a saturation region and a linear region.

[0007] Linear region is a region of which current value changesaccording to the voltage between the source and drain (V_(ds) ) and thevoltage between the gate and source (V_(gs))·(|V_(ds)|<|V_(gs)−V_(Th)|)In the linear region, the following expression (1) is established. Notethat I_(ds) is the amount of current running through a channel formingregion. Note also that β=μC_(o)·W/L is established and μ thereof is amobility of the driver transistor, C_(o) is a gate capacity per unitvolume, and W/L is a ratio of the channel width W to the length L inchannel forming region.

I_(ds)=β{(V_(gs)−V_(Th))V_(ds)−1/2·V_(ds) ²} . . .  (1)

[0008] According to the expression (1) above, V_(ds) and V_(gs) obtainthe current value in the linear region. In the linear region, the lesserV_(ds) becomes, the lesser current value becomes too, while the currentvalue hardly increases even if V_(gs), gets larger.

[0009] When the driver transistor is operated in mainly the linearregion, the amount of current flowing between both electrodes of thelight emitting element is changed according to both values of V_(gs),and V_(ds) . The driver transistor is used as a switch, and a powersource line and the light emitting element are shorted if necessary,thereby flowing a current into the light emitting element. The currentvalue flowing through the light emitting element is directly influencedby the characteristics (variation and deterioration in the manufacturingprocess) of the light emitting element that is connected to the drivertransistor.

[0010] In the saturation region, the current value is not changed by thevoltage between the source and drain (V_(ds)), in other words, it isonly changed by the voltage between the gate and source(V_(gs))·(|V_(ds)|>|V_(gs)−V_(Th)|)

[0011] In the saturation region, the following expression (2) isestablished.

I_(ds)=β(V_(gs)−V_(Th))² . . .   (2)

[0012] As set forth in the expression (2), the current value in thesaturation region is greatly dependent on a change in V_(gs), but notdependent on a change in V_(ds). Therefore, the current value in thesaturation region is not influenced by the characteristics of the lightemitting element connected to the driver transistor.

[0013] On the other hand, when the driver transistor is operated inmainly the saturation region, the amount of current flowing between bothelectrodes of the light emitting element is greatly dependent on achange in V_(gs) of the driver transistor but not dependent on a changein V_(ds) . A gate voltage of the driver transistor is controlled toflow the necessary amount of current into the light emitting element. Inother words, the driver transistor is used as a voltage control currentsource and the driver transistor is set such that a constant currentflows between a power source line and the light emitting element.

[0014] In the constant current drive utilizing the abovementionedfeature, the current value is not dependent on a change in V_(ds) whenthe driver transistor is operated in the saturation region. Therefore,the amount of current flowing into the light emitting element can beconstant regardless of the characteristics (variation in themanufacturing process, deterioration, and temperature variation) of thelight emitting element.

[0015] When V_(gs) of a driver transistor is changed appropriately, thedriver transistor can be operated in mainly a linear region or in mainlya saturation region.

[0016] Operating a driver transistor in the saturation region as shownabove is disclosed in patent document 1.

[0017] [Patent Document]

[0018] Japanese Patent Laid-Open No. Hei 14-108285

[0019] In the abovementioned constant current drive, an operating regionof a transistor steps into the linear region once V_(ds) thereof isdecreased to a certain point by the deterioration of an light emittingelement. To avoid this, a setting voltage of V_(ds) (V_(ds) of a drivertransistor in operation) is set with an estimated deterioration (voltagefor deterioration, voltage a) of the light emitting element. The voltagea is dependent on the deterioration of the light emitting element.

[0020] In a conventional setting voltage, in short, V_(ds) needed to beset high because of the estimated value (812) for the margin of thechange in characteristics of a light emitting element between before(810) and after (811) deterioration. (|V_(ds)|≧V_(gs)−V_(Th)+α|)

[0021] The voltage applied to the cathode and anode of a light emittingelement thus became inevitably high, causing heat generation and highpower consumption.

[0022] It is an object of the invention to provide a pixel structurewhich can be operated without adding the voltage a to the settingvoltage for the deterioration of the light emitting element. Namely, apixel structure with the setting voltage in the vicinity of the boundarybetween the saturation region and the linear region (813 in FlG. 8) isto be provided. A further object of the invention is to provide adisplay device provided with an aforementioned pixel and a controlmethod thereof.

SUMMARY OF THE INVENTION

[0023] The present invention has been made in view of the aboveproblems, and has an object thereof to modify a setting voltage byproviding a power source voltage controller which feedbacks the changein current value in accordance with the deterioration of the lightemitting element and sets the setting voltage thereby. Therefore, thesetting voltage in the vicinity of the boundary between the saturationregion and the linear region is to be provided, without the margin ofvoltage a for the deterioration particularly in the initial settingvoltage.

[0024] To put it concretely, the invention utilizes an element to checkthe deterioration of a light emitting element (hereinafter referred toas a monitoring element) and controls the power source voltage inaccordance with the deterioration of the monitoring element. That is,voltage between the source and drain is modified to a constant value byfixing the potentials of a gate electrode and source electrode of thedriver transistor of the monitoring element and controlling thepotential of a drain electrode (drain terminal) in accordance with thedeterioration of the light emitting element.

[0025]FIG. 1 is a pattern diagram of the structure of the invention,showing a pixel portion 103 having a monitoring element 101 and a pixel102. The monitoring element 101 has a light emitting element and adriver transistor connected to the pixel. The pixel 102 also has a lightemitting element and a driver transistor connected to the pixel. Theinvention has a first electrode 104 and a second electrode 105 connectedto the monitoring element 101 and the pixel 102. A potential of thefirst electrode is shown as V₁, and a potential of the second electrodeis shown as V₂. Note that a monitoring element may be set up at any partincluding outside of the pixel portion.

[0026] Furthermore, the invention has a power source voltage controller106 so as to keep the current value constant by recognizing the changein current value in accordance with the deterioration of a monitoringelement. Namely, the change in current value with the deterioration ofthe monitoring element 101 is fed back to the power source voltage ofthe pixel, fixing the potential of the first electrode: V₁, and changingthe potential of the second electrode: V₂. As the second electrode 105is connected to the monitoring element 101 and the pixel 102, currentvalue of the pixel 102 is kept constant by changing V₂.

[0027] Concerning FIG. 1, a layout of the pixel and monitoring elementand the structure of the elements are to be identical, while connections(with or without connections) may vary. Concerning the invention,however, the structures of the pixel and the monitoring element do notnecessarily have to be identical. However, in the case of forming themonitoring element with the identical structures and differentconnections, manufacture thereof can be easier as there is no need tochange the process but only the design of contacts and the like need tobe changed.

[0028] Operation to control the power source voltage is now explainedwith reference to the flow chart, FIG. 2.

[0029] First, voltage to apply to the light emitting elements of themonitoring element and a pixel is set (driving voltage of light emittingelements). At this time,. the driver transistor is set to operate in thesaturation region, but the deterioration margin (voltage α) is notnecessarily needed. That is, the voltage α which was conventionallynecessary can be unnecessary or reduced according to the invention.

[0030] After that, a signal is inputted to the monitoring element andthe light emitting element of the pixel to emit light. The gradationexpression method to express multicolor at pixels may be either timegradation method or analog gradation method.

[0031] The light emitting element of the pixel as well as of themonitoring element deteriorate gradually as time passes. As the lightemitting elements of the pixels at this time are expressing gradations,few of them emit light constantly. On the other hand, a light emittingelements of the monitoring elements are controlled to emit light at alltime. That is, the light emitting element of the monitoring elementdeteriorates the fastest. Taking that into account, the power sourcevoltage is controlled to set the setting voltage in accordance with thedeterioration of the light emitting element of the monitoring element.In this way, the setting voltage can be modified in consideration of thedeterioration of the light emitting element of the pixels.

[0032] Deterioration of the light emitting element of the monitoringelement raises the resistance value of the light emitting element,lowers I_(ds) of the driver transistor, and reducesV_(ds of the driver transistor. At this time, the setting voltage is to be adjusted by a power source voltage controller to bring the current value to the setting current. That is, V)₂ is to be reduced and the voltage applied to the light emitting elementis to be raised. Furthermore, the monitoring element and the pixel havethe same V₂, so that the setting voltage of the pixel is modifiedsimultaneously.

[0033] It is to be noted that in the invention, the power source voltagemay be controlled by recognizing the change in voltage value or thecharacteristics in accordance with the deterioration of the monitoringelement. The power source voltage may also be controlled by the otherchanges besides the changes in voltage value and current value inaccordance with the deterioration of the monitoring element.

[0034] As described above, the invention enables the driver transistorto operate in the saturation region without adding the deteriorationmargin (voltage α) to the setting voltage when the light emittingelement starts emitting light. Therefore, the margin of the settingvoltage due to the deterioration of the light emitting element is notneeded anymore. In general, the voltage a for the deterioration marginis estimated at 2 to 6V, which causes the driving voltage to decrease asmuch. As a result, heat generation and high power consumption at pixelscan be avoided. As heat generation of the driver transistor can bereduced particularly, the deterioration of the light emitting elementcan be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

[0035]FIG. 1 is a pattern diagram of a pixel portion of the invention.

[0036]FIG. 2 is a flow chart showing an operation of a display device ofthe invention.

[0037]FIGS. 3A and 3B are pattern diagrams of a pixel portion of adisplay device of the invention.

[0038]FIG. 4 is a pattern diagram of a pixel portion of a display deviceof the invention.

[0039]FIG. 5 is an equivalent circuit diagram of a pixel portion of adisplay device of the invention.

[0040]FIG. 6 is a top plan view of a pixel portion of a display deviceof the invention.

[0041]FIGS. 7A and 7B are top plan views of a display module of theinvention.

[0042]FIG. 8 is a view showing a V-I feature of a transistor.

[0043]FIGS. 9A to 9H are views showing electronic appatatuses havingpixel portions of display devices of the invention.

[0044]FIG. 10 is a pattern diagram of a pixel portion of a displaydevice of the invention.

[0045]FIGS. 11A and 11B are pattern diagrams of a pixel portion of adisplay device of the invention.

[0046]FIG. 12 is a view showing an experimental circuit of theinvention.

[0047]FIG. 13 is a chart showing a change of an anode potential(V_(cathode)) with the passage of time (hour).

[0048]FIG. 14 is a chart showing a current value (I) which is suppliedto a light, emitting element with the passage of time (hour).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] Preferred embodiments of the invention will be hereinafterdescribed referring to the accompanying drawings. Note that an activeelement has a gate, a source and a drain, but it is impossible todistinguish a source electrode and a drain electrode because of theelement structures. Therefore, when a connection between the terminalsis explained, either of the source electrode or the drain electrode isreferred to as a first electrode, and the other is referred to as asecond electrode for convenience.

[0050] [Embodiment Mode 1]

[0051] Hereinafter explained with reference to FIG. 3 is an example inwhich an operational amplifier is used as a power source voltagecontroller. Note that in this embodiment mode, a p-channel type drivertransistor is applied, but an n-channel type driver transistor can beapplied instead. Meanwhile, as an example of the invention, an anodepotential of a pixel 311 (V_(anode)) is referred to as V₁, and cathodepotentials of the pixel 311 and of a monitoring element 301(V_(cathode)) are referred to as V₂.

[0052]FIG. 3A shows an equivalent circuit diagram of a pixel structureof the invention. A monitoring element 301 has a driver transistor 302and a light emitting element 303 which is connected to a secondelectrode of the driver transistor 302. A pixel 311 has a drivertransistor 312 and a light emitting element 313 which is connected to asecond electrode of the driver transistor 312. The light emittingelements 303 and 313 are connected to an output terminal of theoperational amplifier 320 and have a voltage ofV_(2. Meanwhile, first electrodes of the driver transistors 302 and 312 have potential V)₁ which is the same potential as the electrodes of the light emittingelements.

[0053] A non-inverted input terminal (+side) of the operationalamplifier 320 is connected to the first electrode of the drivertransistor 312 of the pixel, and an inverted input terminal (−side) isconnected to the first electrode of the driver transistor 302 of themonitoring element. The driver transistor of the monitoring element isconnected to a reference power source (V_(ref)) via a resistance (R).Note that V_(ref) is higher than V₁ (V_(anode)).

[0054] Next, a method for setting a voltage of the driver transistor ofthe monitoring element (V_(ds)) is explained.

[0055] First, V₁ (namely, V_(anode) of the monitoring element) is set onthe basis of a specification of a display device. Specifications ofgeneral display devices prescribe V₁ as 2 to 6V. A gate voltage of thedriver transistor 302 (V_(moni)) is set so that a predetermined current(I_(ref)) can flow in the monitoring element in a saturation region.Then, a gate voltage of the driver transistor 312 (V_(pix)) is set atthe same value or higher than V_(moni).

[0056] Next, the reference power source V_(ref) and the resistance valueR are set so that the predetermined current (I_(ref)) can flow in themonitoring element 301 and the driver transistor 302 can operate in asaturation region. Note that other means to supply a predeterminedcurrent which flows in the monitoring element can be used besides thereference power source V_(ref) and the resistance R. For example, thepredetermined current (I_(ref)) can be supplied by a current source 321as shown in FIG. 3B.

[0057] Namely, the values of V₁, V_(ref),V_(moni), V_(pix) and R aredetermined by a practitioner and I_(ds) of the driver transistor 302 ofthe monitoring element is controlled externally.

[0058] As described above, a display operation is started by setting themonitoring element (called an operation state or a driving state). Afterthat, the light emitting element 303 of the monitoring elementdeteriorates as time passes. Similarly, the light emitting element 313of the pixel deteriorates. Further, due to the deterioration of thelight emitting element, the resistance value becomes high, thus acurrent value flowing in the monitoring element becomes low.

[0059] The operational amplifier will be hereinafter explained. Theoperational amplifier 320 basically functions with an input potential ofapproximately 0 and with a non-inverted input terminal and an invertedinput terminal, the voltages of which are approximately the same.Accordingly, the following expression is established.I_(ref) = (V_(ref) − V₁)/R = (V₁ − V₂)/R_(moni)∴ V₂ = (R_(moni)/R + 1) ⋅ V₁ − (R_(moni)/R) ⋅ V_(ref)

[0060] R_(moni) is a resistance value between the first electrode of thedriver transistor 302 and the power source side of the light emittingelement 303. The operational amplifier changes the value of V₂ whenR_(moni) is changed and keeps the I_(ref) value constant.

[0061] Such an operational amplifier modifies a setting voltage andfurther modifies V_(ds) of the driver transistor of the pixel becausethe monitoring element and the pixel has V₂ (V_(cathode)) in common.

[0062] Meanwhile, in this embodiment mode, V_(ds) is set in a saturationregion even when electric characteristics of the driver transistor arechanged by the temperature or the like because I_(ds) is determined andcontrolled by the operational amplifier.

[0063] Hereinafter, the setting current of the monitoring element willbe complimented in view of the difference of the deteriorating speedsbetween the pixel and the light emitting element.

[0064] In the case of a digital gradation method for example, the lightemitting element of the pixel repeatedly performs the light emission andnon-light emission (erasure) on the basis of a signal current (videosignal). Meanwhile, the light emitting element of the monitoring elementconstantly emits light. Therefore, the light emitting element of themonitoring element deteriorates faster than that of the pixel. Thatmeans the deterioration of the light emitting element of the monitoringelement is the fastest of all.

[0065] Namely, a voltage with an estimated deterioration of the lightemitting element can be set in the driver transistor of every pixel whenV_(ds) of the driver transistor is set by the operational amplifier sothat the deterioration of the light emitting element of the mostdeteriorated monitoring element can be offset. Therefore, it is notrequired to control the light emitting element of the monitoring elementcorresponding to the light emission and non-light emission of the lightemitting element of the pixel.

[0066] However, preferably required is the case where a light emissionratio of the light emitting element of the pixel per frame (duty ratio)is figured, and the light emitting element of the monitoring element aremade to emit light in accordance with the duty ratio. Namely, in thecase of the digital gradation system, the setting current of themonitoring element is preferably set at (current value during the lightemission x duty ratio).

[0067] In the case of an analog gradation method, the gradation iscontrolled by the amount of the current which flows into the lightemitting element as described above. Therefore, in the analog gradationsystem, a current value over the average of the pixel with the maximumlight emission is preferably required.

[0068] That is to say, according to the invention, it is possible toobtain a setting voltage with an estimated deterioration of the lightemitting elements of all the pixels by measuring the deterioration ofthe monitoring elements and setting the setting voltage. of themonitoring elements in the saturation region.

[0069] Further, the deterioration ratio of the light emitting elementsis different depending on the materials: red (R), green (G) and blue(B). In this case, by taking the deterioration of the light emittingelements of the monitoring elements equal to or more than the mostdeteriorated element of each light emitting element, V₂ (V_(cathode))can have a sufficient value.

[0070] As above, according to the invention, it is possible to obtain asetting voltage without a deterioration margin (voltage a). Accordingly,a margin of the setting voltage in accordance with a deterioration of alight emitting element is not required, thus heat generation and powerconsumption can be reduced. Particularly, by the reduction in powerconsumption of the driver transistor, the deterioration of the lightemitting element can be prevented.

[0071] [Embodiment Mode 2]

[0072] Hereinafter explained with reference to FIG. 4 is a differentpixel structure from that of Embodiment Mode 1. Note that in thisembodiment mode, a driver transistor is a p-channel type, an anodepotential of a light emitting element (V_(anode)) is referred to as V₁,and a cathode potential of the light emitting element (V_(cathode)) isreferred to as V₂.

[0073] An equivalent circuit diagram of a pixel structure is shown inFIG. 4. As well as Embodiment Mode 1, a monitoring element 401 has adriver transistor 402 and a light emitting element 403 which isconnected to a second electrode of the driver transistor 402. A pixel411 has a driver transistor 412 and a light emitting element 413 whichis connected to a second electrode of the driver transistor 412. Thelight emitting elements, 403 and 413 are connected to an output terminalof the operational amplifier 420 and have a voltage of V₂. Meanwhile,first electrodes of the driver transistors 402 and 412 have potential V₁which is the same potential as the electrodes of the light emittingelements.

[0074] Unlike Embodiment Mode 1, a non-inverted input terminal (+side)of the operational amplifier 420 is connected to a bias power sourceV_(b), and an inverted input terminal (−side) is connected to theinterconnection between the driver transistor of the monitoring element402 and the light emitting element 403.

[0075] Next, a method for setting a voltage of the driver transistor ofthe monitoring element (V_(ds)) is explained.

[0076] First, V₁ is set on the basis of a specification of a displaydevice. Then, a gate voltage of the driver transistor 402 (V_(moni)) isset so that a predetermined current (I_(ref)) can flow in the monitoringelement in a saturation region. Also, a gate voltage of the drivertransistor 412 (V_(pix)) is set at the same value as or higher thanV_(moni).

[0077] Next, V_(b) is determined so that the driver transistor 402 ofthe monitoring element can operate in a saturation region. Namely,V_(ds of the driver transistor is determined. As described above, a current which flows in the monitoring element 401 is determined by the operational amplifier and V)₂ (V_(cathode)) is determined so that the current which flows in themonitoring element 401 can flow in the light emitting element 403.

[0078] Namely, the values of V₁ and V_(b) are determined externally andV_(ds) of the driver transistor of the monitoring element is controlled.

[0079] When a display operation is started by setting as above, aresistance value rises due to the deterioration of the light emittingelement. Then, with the current value flowing in the monitoring elementlowered, V_(ds) of the driver transistor 402 also tries to lower itself.However,V_(ds is held constant because the potential difference between the input terminals of the operational amplifier is ideally)0. Then, I_(ds) becomes constant because V_(gs) and V_(ds) are constant,thus V₂ is automatically selected by I_(ds).

[0080] The monitoring element and the pixel has V₂ (V_(cathode)) incommon. That means V_(ds) of the driver transistor of the pixel is setas well.

[0081] In this manner, this embodiment mode is characterized bydetermining V_(ds). Further, this embodiment mode in which V_(ds) isdirectly controlled provides a simpler method for setting a voltage ascompared to Embodiment Mode 1 in which I_(ds) is determined.

[0082] As described above, according to the invention, it is possible toobtain a setting voltage without a deterioration margin (voltage a) fromthe time the light emitting element starts emitting light. Therefore, a.margin of the setting voltage in accordance with a deterioration of thelight emitting element is not required, thus heat generation and powerconsumption can be reduced. Particularly, by the reduction in heatgeneration of the driver transistor, the deterioration of the lightemitting element can be prevented.

[0083] [Embodiment Mode 3]

[0084] Hereinafter explained is a different pixel structure from thoseof Embodiment Mode 1 and Embodiment Mode 2.

[0085]FIG. 10 shows a circuit diagram as described in Embodiment Mode 2,wherein a switching regulator 3000 is applied in stead of an operationalamplifier as a power source voltage controller. For a constitution wherean operational amplifier is applied, a power source circuit for theoperational amplifier is required. This embodiment mode makes itpossible to unite an operational amplifier and the power source circuitby using a switching regulator.

[0086] A pixel structure which has a switching regulator will behereinafter described. In FIG. 10, the switching regulator 3000 iscomprised of an error amplifier 3001, a PWN comparator 3002, a referencepower sources 3003 and 3010, an oscillation circuit 3004, a switchingtransistor 3008, an inductor 3009, a diode 3006, a smoothing capacitor3005 and a battery 3007. As well as Embodiment Mode 2, a monitoringelement has a driver transistor 3011 and a light emitting element 3012which is connected to a first electrode of the driver transistor 3011. Apixel has a driver transistor 3013 and a light emitting element 3014which is connected to a first electrode of the driver transistor .3013.Gate electrodes of the driver transistors 3011 and 3013 are connected toa power source 3015 and second electrodes of the transistors 3011 and3013 are connected to a power source 3016.

[0087] Next, an operation of the switching regulator will be explained.At the start of operating, a potential of the smoothing capacitor 3005which is an output of the switching regulator is 0. The potential of thesmoothing capacitor is inputted to an inverted input terminal of theerror amplifier 3001, and a potential of the light emitting element isinputted to a non-inverted input terminal. A current of the transistor3011 flows in the light emitting element 3012 and a voltage is generatedin the light emitting element. When the voltage is higher than that ofthe reference. power source. 3010, the error amplifier 3001 operates soas to lower the output. Then, the. PWN comparator 3002 operates so as tolower the voltage of the inductor 3009 by changing the duty of theoscillation. Therefore, a potential of the smoothing capacitor 3005 islowered and an anode potential of the light emitting element 3012 isalso lowered to become approximately the same potential as the powersource 3010. Meanwhile, when an anode potential of the light emittingelement 3012 is lower than that of the reference power source 3010, theopposite operation is taken, and the anode potential rises to the samepotential as the reference power source 3010.

[0088] In this manner, the same effect as that of an operationalamplifier can be obtained by using the switching regulator 3000. Also, apower source can be reduced.

[0089] [Embodiment Mode 4]

[0090] Hereinafter explained with reference to FIGS. 5 and 6 is thepixel portion having a monitoring element. Note that a thin filmtransistor (hereinafter referred to as TFT) formed over an insulatingsurface is employed as a transistor of an active element in thisembodiment mode.

[0091] Shown in FIG. 5 is an equivalent circuit diagram of a pixelportion 500 having a first dummy pixel 501, a monitoring element 502, asecond dummy pixel 503 and a pixel 504 in this order. The first andsecond dummy pixels are provided so that the whole pixel portion isunder the equal condition including the pixels at the edge as well asthe pixels around them.

[0092] The dummy pixel, the monitoring element and the pixel havesimilar structures, having a first TFT (selector TFT) 511, a second TFT(erasure TFT) 512, a third TFT (driver TFT) 513, a capacitor element514, and a light emitting element 515 at the crossed part of a signalline 521 and a first scanning line 522. Note that the selector TFT andthe erasure TFT are formed by using n-channel type TFTs, and the driverTFT is formed by using a p-channel type TFT in this embodiment mode.Also, a second scanning line 523 which is connected to a gate electrodeof the erasure TFT, and a current supply line 524 which is connected toa first electrode of the erasure TFT and a first electrode of the driverTFT are provided.

[0093] The dummy pixel, the monitoring element and the pixel, however,vary in connections of each structure. First and second dummy pixels arenot connected to the first electrode of the selector TFT 511 and thesignal line 521. Secondly, the first electrode of the driver TFT 513 isnot connected to the first electrode of the light emitting element 515.These dummy pixels are provided in order to operate the whole pixelsunder the same condition including the pixels at the edge as well as thepixels around them. Therefore, dummy pixels neither need to emit light,write data from the signal line to pixels, nor make the light emittingelement emit light. In the invention, however, the dummy pixel may emitlight. The signal line 521 of the dummy pixel and the current supplyline 524 are connected to each other to have the same potentials.

[0094] In the monitoring element, the first electrode of the selectorTFT 511 is not connected to the signal line 521. The signal line 521,however, is connected to the first electrode of the light emittingelement 515. This is intended to make the monitoring element emit lightconstantly so that the deterioration thereof proceeds fast. Therefore,voltage applied from the signal line as information for brightness doesnot have to go through the selector TFT 511 to be inputted to the lightemitting element. The signal line 521 and the current supply line 524 ofthe monitor element are connected to an operational amplifierrespectively.

[0095] In the pixel, the first electrode of the selector TFT 511 isconnected to the signal line 521, and a first electrode of the driverTFT 513 is connected to the first electrode of the light emittingelement 515. It is intended that in the pixel, the light emittingelement 515 emits light through the driver TFT 513 based on the signalvoltage from the signal line. Furthermore, the signal line 521 and thecurrent supply line 524 of the pixel are connected to the driver circuitand an FPC respectively.

[0096] In FIG. 6, a top plan view of a part of the pixel portion shownin FIG. 5 is shown. The first dummy pixel 501, the monitoring element502, the second dummy pixel 503, and the pixel 504 of the first line areshown. These dummy pixels, monitoring elements and pixels have theselector TFT 511, the erasure TFT 512, the driver TFT 513, and the lightemitting element 515 (only the first electrode thereof is shown) at thecrossed parts of the signal line 521, the current supply line 524, thefirst scanning line 522, and the second scanning line 523. A capacitorelement 514 (configured with a gate metal and a semiconductor film ofthe TFT 513) is provided as needed. Note that, another capacitor elementis added when the gate capacitor of the driver TFT is too small for theleakage current of the TFT.

[0097] As described above with reference to FIG. 5, these dummy pixels,the monitoring element, and the pixel have the identical structures,however, the presence of contacts differs. That is, what differs in thedummy pixels, the monitoring element, and the pixel is whether theconnection between the selector TFT 511 and the signal line 521 and theconnection between the driver TFT 513 and the light emitting element 515exist or not.

[0098] In the first and second dummy pixels, there are no contacts inthe contact portion of the selector TFT 511 and the signal line 521, andin the contact portion of the driver TFT 513 and the first electrode ofthe light emitting element 515. The monitoring element, however, isprovided with a contact 601 with the signal line 521 in the contactportion of the driver TFT 513 and the light emitting element 515,although there is no contact in the contact portion of the selector TFT511 and the signal line 521. In the pixel, there is a contact 602 in thecontact portion of the selector TFT 511 and the signal line 521, and acontact 603 in the contact portion of the driver TFT 513 and the lightemitting element 515.

[0099] Furthermore, a leading wiring is provided so that the signal lineand the current supply line of the monitoring element are connected tothe operational amplifier. Moreover, the signal line and the currentsupply line of the pixel. are connected to an FPC terminal 506 or adriver circuit respectively. The signal line and the current supply lineof the dummy pixel are connected to each other and have the samepotentials.

[0100] The monitoring element is not necessarily required in a wholeline, but has only to be provided one. It depends on the performance ofthe- operational amplifier to which the monitoring element is connected.The monitoring elements may be provided in plural, and also can bedisposed symmetrically to the pixel portion. The monitoring element maybe disposed in any forms.

[0101] A line of monitoring elements are connected to each other inparallel through the current supply line, and a plurality of monitoringelements can be seen as one big monitoring element.

[0102] In this manner, the monitoring element of the invention can beformed by changing the layout design of the element, without changingthe process of the pixel. Also, the setting voltage of the pixel can beat the best voltage in the saturation region at all times by utilizingthe monitoring element formed thereby. Therefore, heat generation andpower consumption can be reduced, resulting in the longer life of thelight emitting element.

[0103] [Embodiment Mode 5]

[0104] The pixel portion shown in the above embodiment mode is providedlight emitting elements and sealed not to be exposed to the air, thuscompleting a panel. ICs including an operational amplifier, acontroller, and a power source circuit and the like are mounted on thepanel, thus completing a display module. The specific structure of thedisplay module is explained here.

[0105] A pattern diagram provided with a line of monitoring elements 751near the signal line driver circuit 705 is shown in FIG. 11A. Themonitoring element shown in FIG. 11A comprises a signal line drivingcircuit 4001, a scanning line driving circuit 4002, a plurality of dummypixels 4003, a plurality of pixels 4004 and a monitoring element 4005.The invention has monochrome light emitting elements when the monitoringelements are provided in one line as shown in FIG. 11A. Therefore, it isdesirable to apply it to a display device which expresses RGB with acolor converting layer.

[0106] Furthermore, the monitoring elements may be provided in aplurality of lines or at a plurality of portions as shown in FIG. 11B.The monitoring element shown in FIG. 11A comprises a signal line drivingcircuit 4001, a scanning line driving circuit 4002, a plurality of dummypixels 4003, a plurality of pixels 4004, a 1st monitoring element (R)4006, a 1st monitoring element (G) 4007 and a, 1st monitoringelement.(B) 4008. When providing the monitoring elements in a largepanel particularly, it is desirable that the monitoring elements beprovided in a plurality of lines although it depends on the performanceof the operational amplifier. At the same time, first to thirdmonitoring elements had better be provided in consideration of thedifference of deteriorations between the materials for each color (RGB)as shown in FIG. 11B. The place to dispose each monitoring element isnot exclusively applied to FIG. 11B. It may be disposed at anyperipheral region of the pixel, including outside thereof.

[0107] An outline view of the display module of the structure of FIG.11A is shown in FIG. 7A. The display module is mounting an operationalamplifier 750, a controller 701 and a power source circuit 702. A pixelportion 703 in which a light emitting element is formed in each pixel, amonitoring element 751 and a dummy pixel 752 are mounted, a scanningline driver circuit 704 for selecting pixels in the pixel portion 703,and a signal line driver circuit 705 for supplying a video signal to theselected pixels are formed in the panel 700. The monitoring element 751is disposed near the signal line driver circuit 705 as one side of thepixel portion 703 and connected to the operational amplifier 750. Inaddition, the dummy pixel 752 is provided around (periphery of) thepixel portion 703 in order to put the pixels at the edge (the mostoutside pixels. In case of m x n pixels, and a pixel in the first rowand the first line, a pixel in the m-th row and the n-th line) under thesame condition as the peripheral pixels.

[0108] Similarly, the dummy pixel 752 may be disposed at the necessaryperiphery only although it is disposed around the pixel portion 703 inthe figure. Also, the place and number of the signal line driver circuitand the scanning line driver circuit to dispose are not limited to FIG.7A.

[0109] Further, the operational amplifier 750, the controller 701 andthe power source circuit 702 are formed over a printed circuit board706. Each type of signal and a power source voltage outputted from thecontroller 701 or the power source circuit 702 are supplied through anFPC 707 to the pixel portion 703, the scanning line driver circuit 704,and the signal line driver circuit 705 of the panel 700.

[0110] The power source voltage and each type of signal are supplied tothe printed circuit board 706 through an interface (I/F) portion 708 onwhich a plurality of input terminals are disposed.

[0111] Note that although the printed circuit board 706 is mounted viaan FPC 707 on the panel 700 in this embodiment mode, the structure isnot necessarily limited to this. The controller 701 and the power sourcecircuit 702 may also be mounted directly on the panel 700 by using a COG(Chip on Glass) method.

[0112] Further, noise may ride on the power source voltage and thesignals, and the signal rise time may become slowed, due to capacitancethat are formed between leading wirings, resistance of wiringsthemselves, and the like of the printed circuit board 706. Various typesof elements, such as capacitors and buffers, may be formed over theprinted circuit board 706 so. as to prevent noise from riding on thepower source voltage or the signals, and slowness in the signal risetime.

[0113] A block diagram of the structure of the printed circuit board 706is shown in FIG. 7B. Each type of signal and the power source voltagesupplied by the interface 708 are supplied to the controller 701 and thepower source circuit 702.

[0114] The controller 701 has an A/D converter 709, a phase locked loop(PLL) 710, a control signal generator portion 711, and SRAMs (StaticRandom Access Memories) 712 and 713. Note that, although SRAMs are usedhere, it is also possible to use SDRAMs or DRAMs (Dynamic Random AccessMemories) as substitutes for the SRAMs, provided that the DRAMs arecapable of writing and reading data at a high speed.

[0115] A video signal supplied through the interface 708 is subjected toa parallel-serial conversion in the A/D converter 709, and inputted tothe control signal generator portion 711 as a video signal correspondingto the colors R, G. and B (R video signal 714, G video signal 715 and Bvideo signal 716). Further, an Hsync signal 717, a Vsync signal 718, aclock signal CLK (CLK1 719 and CLK2 720), and alternating voltage (ACCont 721) are generated in the A/D converter 709 based on each of thesignals supplied through the interface 708, and then inputted to thecontrol signal generator portion 711.

[0116] The phase locked loop 710 functions to align the phase of theoperating frequency of the control signal generator portion 711 with thefrequency of each of the signals supplied through the interface 708. Theoperating frequency of the control signal generator portion 711 is notnecessarily the same as the frequency of each of the signals suppliedthrough the interface 708. The operating frequency of the control signalgenerator portion 711 is regulated in the phase, locked loop 710 so thatthe frequencies become synchronized.

[0117] The video signal inputted to the control signal generator portion711 is temporarily written into the SRAMs 712 and 713, and stored. Avideo signal corresponding to all pixels is read out one bit at a timefrom the video signals of all of the bits that are stored in the SRAM712, and then supplied to the signal line driver circuit 705 of thepanel 700.

[0118] The control signal generator portion 711 supplies information,which relates to light emission periods by the light. emitting elements.for each bit, to the scanning line driver circuit 704 of the panel 700.

[0119] The power source circuit 702 supplies a predetermined powersource voltage to the signal line driver circuit 705, the scanning linedriver circuit 704, and the pixel portion 703 of the panel 700.

[0120] The display module formed like this can set the best settingvoltage in the saturation region all the time by utilizing themonitoring element. Therefore, heat generation and power consumption canbe. reduced, resulting in the longer life of the light emitting element.

[0121] [Embodiment Mode 6]

[0122] Given as examples of electronic apparatuses that employ displaydevices manufactured in accordance with the invention are video cameras,digital cameras; goggle type displays (head mounted displays),navigation systems, audio playback devices (car audios, audiocomponents, etc.), notebook type personal computers, game machines,portable information terminals (mobile computers, mobile telephones,mobile type game machines, and electronic books, etc.), imagereproduction devices equipped with a recording medium (specifically,devices equipped with a display device capable of reproducing therecording medium such as a Digital Versatile Disk (DVD), etc. anddisplaying the image thereof), and the like. Examples of theseelectronic apparatuses are shown in FIG. 9.

[0123]FIG. 9A is a display device, which is composed of a frame 2001, asupport base 2002, a display portion 2003, a speaker portion 2004, avideo input terminal 2005, and the like. The pixel portion having amonitoring element of the invention is used for the display portion 2003to manufacture the display device. Note that in the case ofmanufacturing a large-sized display device, the monitoring elements areprovided in a plurality of rows or more preferably at every RGBs. Whenthe invention is applied to such a display device, particularly to alarge-sized display device, the low power consumption is achieved, thusthe problems of the heat generation and the deterioration of the lightemitting elements can be solved. Note that the term display deviceincludes all display devices for displaying information, such as thosefor personal computers, those for receiving TV broadcasting, and thosefor advertising.

[0124]FIG. 9B is a digital still camera, which is composed of a mainbody 2101, a display portion 2102, an image-receiving portion 2103,operation keys 2104, an external connection port 2105, a shutter 2106,and the like. The pixel -portion having a monitoring element of theinvention is used for the display portion 2102 to manufacture thedigital still camera.

[0125]FIG. 9C is a notebook type personal computer, which is composed ofa main body monitoring element of the invention is used for the displayportion 2602 to manufacture the video camera.

[0126]FIG. 9H is a mobile telephone, which is composed of a main body2701, a frame 2702, a display portion 2703,.an audio input portion 2704,an audio output portion 2705, 5. operation keys 2706, an externalconnection port 2707, an antenna 2708, and the like. The pixel portionhaving a monitoring element of the invention is used for the displayportion 2703 to manufacture the mobile telephone., Note that bydisplaying white characters on a black background, the display portion2703 can suppress power consumption of the mobile telephone.

[0127] As described above, the application scope of the invention is sowide that it can be used in electronic apparatuses of various fields,particularly to a flat panel display.

[0128] [Embodiment]

[0129] [Embodiment 1]

[0130] Hereinafter explained is an experimental result with respect toa. change of a cathode voltage (cathode potential) corresponding to asecular change and a current which is supplied to a light emittingelement. Incidentally, this embodiment mode applies an experimentalcircuit to which a circuit as shown in FIG. 10 is applied.

[0131]FIG. 12 shows a circuit diagram of this embodiment to which acircuit diagram of FIG. 10 is applied., For a power source circuit, aμPC1100 (produced by NEC Corporation) is used. A resistance R1 is set sothat a voltage of a VMO terminal can be approximately 1.05 to 1.45V. Aresistance R2 is set so that a driver TFT of a monitoring element canoperate in a saturation region. A resistance R3 is set so that a voltageof a DTC terminal can be approximately 1.87V. A power source voltage Vccis set at a voltage of 7V. A CATHODE terminal is connected to a cathodeof a light emitting element of a pixel and a cathode of a light emittingelement of the monitoring element. A MONITOR terminal is connected to ananode .of the light emitting element of the monitoring element.Meanwhile, second electrodes of driver TFTs of the pixel and themonitoring element and which are not connected to the light emittingelements are fixed at 5V.

[0132] Shown in FIGS. 13 and 14 are the results of experiment conductedby using the circuit as shown in FIG. 12. FIG. 13 shows a change of acathode potential (V_(cathode)) with the passage of time (hour). It canbe confirmed that an absolute value of the cathode potential(V_(cathode)) rises as time passes. Meanwhile, FIG. 14 shows a currentvalue (I) which is supplied to a light emitting element with the passageof time (hour). It can be confirmed that a constant current value issupplied to the light emitting element. Note that the current valuesupplied to the light emitting element is equal to the currentconsumption.

[0133] The longer the light emitting element emits light, the faster thelight emitting element deteriorates and the higher the absolute value ofthe required cathode voltage becomes. However, the current valuesupplied to the light emitting element does not change. Accordingly, thecircuit of the invention can control the cathode voltage normally inorder to keep the current value supplied to the light emitting elementconstant.

[0134] By using the pixel structure provided with the circuit of theinvention, it is possible to operate a diver TFT in a saturation regionwithout a deterioration margin of the light emitting element, or with asmaller margin than the conventional one. Therefore, heat generation andpower consumption can be reduced.

[0135] The invention makes it possible to set a voltage in a saturationregion without a deterioration margin of a light. emitting element, orwith a smaller margin than the conventional one from the time the lightemitting element starts emitting light. Therefore, the margin of thesetting voltage in accordance with the deterioration of the lightemitting element is not required, leading to the reduced heat generationand power consumption. Further, the deterioration of the light emittingelement can be prevented particularly because heat generation. of thedriver transistor is reduced.

What is claimed is:
 1. A display device comprising: a pixel having afirst light emitting element and a first transistor which is connectedto the first light emitting element; a monitoring element having asecond light emitting element and a second transistor which is connectedto the second light emitting element; and. a power source voltagecontroller, wherein the power source voltage controller functions tomodify a voltage between the source and drain of the second transistorto be a constant value.
 2. A display device comprising:. a pixel havinga first light emitting element and a first transistor which is connectedto the first light emitting element; a monitoring element having asecond light emitting element and a second transistor which is connectedto the second light emitting element; and a power source controller toset a voltage between the source and drain of the first transistor and avoltage between the source and drain of the second transistor in thevicinity of the boundary between the saturation region and the linearregion.
 3. A display device comprising: a pixel having a first lightemitting element and a first transistor which is connected to the firstlight emitting element; a monitoring element having a second- lightemitting element and a second transistor which is connected to thesecond light emitting element; and a power source voltage controller toset a voltage between the source and drain of the first transistor inthe vicinity of the boundary between the saturation region and thelinear region by modifying the voltage between the source and drain ofthe second transistor to be a constant value.
 4. A display devicecomprising: a pixel having a first transistor and a first light emittingelement which is connected to a first electrode of the first transistor;a monitoring element having a second transistor and a second lightemitting element which is connected to a first electrode of the secondtransistor; and a power source voltage controller, wherein an inputterminal of the power source voltage controller is connected to secondelectrodes of the first and second transistors respectively; an outputterminal of the power source voltage controller is connected to theelectrodes of the first and second light emitting elements; and afunction to set a voltage between the source and drain of the firsttransistor and a voltage between the source and drain of the secondtransistor in the vicinity of the boundary between the saturation regionand the linear region.
 5. A display device according to claim 2: whereinthe saturation region establishes |V_(ds)|>|V_(gs)−V_(Th)| with respectto a V-I characteristic of the transistor.
 6. A display device accordingto claim 3: wherein the saturation region establishes|V_(ds)>|V_(gs)−V_(Th)| with respect to a V-I characteristic of thetransistor.
 7. A display device according to claim 4: wherein thesaturation region establishes |V_(ds)>|V_(gs)−V_(Th)| with respect to aV-I characteristic of the transistor.
 8. A display device according toclaim 2: wherein the linear region establishes |V_(ds|<|V) _(gs)−V_(Th)|with respect to a V-I characteristic of the transistor.
 9. A displaydevice according to claim 3: wherein the linear region establishes|V_(ds)|<|V_(gs)−V_(Th)| with respect to a V-I characteristic of thetransistor.
 10. A display device according to claim 4: wherein thelinear region establishes |V_(ds)|<|V_(gs)−V_(Th)| with respect to a V-Icharacteristic of the transistor.
 11. A display device comprising: apixel having a first transistor and a first light emitting element whichis connected to a first electrode of the first transistor; a monitoringelement having a second transistor and a second light emitting elementwhich is connected to a first electrode of the second transistor; and apower source voltage controller, wherein an input terminal of the powersource voltage controller is. connected to second electrodes of thefirst and second transistors respectively, and an output terminal of thepower source voltage controller is connected to the electrodes of thefirst and second light emitting elements.
 12. A display devicecomprising: a pixel having a first transistor and a first light emittingelement, wherein a first electrode of the first transistor and a firstelectrode of the first light emitting element are connected to thepixel; a monitoring element having a second transistor and a secondlight emitting element, wherein a first electrode of the secondtransistor and a second electrode of the second light emitting elementare connected to the monitoring element; and a power source voltagecontroller; wherein an input terminal of the power source voltagecontroller is connected to the first electrode of the second transistorelectrically; and an output terminal of the power source voltagecontroller is connected to a second electrode of the first lightemitting element and the second electrode of the second light emittingelement electrically.
 13. A display device comprising: a plurality ofsignal lines and a plurality of scanning lines; a pixel having a firsttransistor which is connected to the signal lines and the scanning linesand a first light emitting element which is connected to a firstelectrode of the first transistor; a monitoring element having a secondtransistor which is connected to the scanning lines and a second lightemitting element which is connected to a first electrode of the secondtransistor; and a dummy pixel having a third transistor which isconnected to the scanning lines and a third light emitting element;wherein the signal lines in the pixel are connected to an FPC; and thesignal lines in the monitoring element are connected to a power sourcevoltage controller.
 14. A display device comprising: a pixel having afirst transistor and a first light emitting element which is connectedto a first electrode of the first transistor;. a monitoring elementhaving a second transistor and a second light emitting element which isconnected to a first electrode of the second transistor; a panelmounting a dummy pixel having a third transistor and a third lightemitting element; and a printed circuit board mounting a power sourcevoltage controller which is connected to the pixel and the monitoringelement, a power source circuit, and a controller, wherein an inputterminal of the power source voltage controller is connected to secondelectrodes of the first and second transistors; and an output terminalof the power source voltage controller is connected to electrodes of thefirst and second light emitting elements.
 15. A display device accordingto claim 1: wherein the power source voltage controller is anoperational amplifier.
 16. A display device according to claim 2:wherein the power source voltage controller is an operational amplifier.17. A display device according to claim 3: wherein the power sourcevoltage controller is an operational amplifier.
 18. A display deviceaccording to claim 4: wherein the power source voltage controller is anoperational amplifier.
 19. A display device according to claim 11:wherein the power source voltage controller is an operational amplifier.20. A display device according to claim 12: wherein the power sourcevoltage controller. is an operational amplifier.
 21. A display deviceaccording to claim 13: wherein the power source voltage controller is anoperational amplifier.
 22. A display device according to claim 14:wherein the power source voltage controller is an operational amplifier.23. A display device comprising: a pixel having a first transistor and afirst light emitting element, wherein a first electrode of the firsttransistor and a first electrode of the first light emitting element areconnected to the pixel; a monitoring element having a second transistorand a second light emitting element, wherein a first electrode of thesecond transistor and a second electrode of the second light emittingelement are connected to the monitoring element; and a switchingregulator; wherein a reference voltage input terminal of the switchingregulator is connected to the first electrode of the second transistorelectrically; and an output terminal of the switching regulator isconnected to a second electrode of the first light emitting element andthe second electrode of the second light emitting element electrically.24. A control method of a display device comprising: a pixel having afirst light emitting element and a first transistor which is connectedto the first light emitting element; a monitoring element having asecond light emitting element and a second transistor which is connectedto the second light emitting element; and a power source voltagecontroller which is connected to the pixel and the monitoring element,wherein the power source voltage controller decides voltages between thesource and drain of the first and second transistors.
 25. A controlmethod of a display device comprising: a pixel having a first lightemitting element and a first transistor which is connected to the firstlight emitting element; a monitoring element having a second lightemitting element and a second transistor which is connected to thesecond light emitting element; and a power source voltage controllerwhich is connected to the pixel and the monitoring element, wherein thepower source voltage controller sets voltages between the source anddrain of the first and second transistors in the vicinity of theboundary between the saturation region and the linear region.
 26. Acontrol method of a display device comprising: a pixel having a firsttransistor and a first light emitting element which is connected to afirst electrode of the first transistor; a monitoring element having asecond transistor and a second light emitting element which is connectedto a first electrode of the second transistor; and a power sourcevoltage controller which is connected to the pixel and the monitoringelement, wherein an input terminal of the power source voltagecontroller is connected to second electrodes of the first and secondtransistors; an output terminal of the power source voltage controlleris connected to electrodes of the first and second light emittingelements; and a current between the source and drain of the secondtransistor is controlled to set a voltage between the source and drainof the first transistor in the vicinity of,the boundary between thesaturation region and the linear region.
 27. A control method of adisplay device according to claim 26: wherein a current between thesource and drain of the second transistor is controlled by a referencepower source and a resistance, or a power source current.
 28. A controlmethod of a display device comprising: a pixel having a first transistorand a first light emitting element, wherein a first electrode of thefirst transistor and a first electrode of the first light emittingelement are connected to the pixel; a monitoring element having a secondtransistor and a second light emitting element, wherein a firstelectrode of the second transistor and a second electrode of the secondlight emitting element are connected to the monitoring element; and apower source voltage controller, wherein an inverted input terminal ofthe power source voltage controller is connected to the first electrodeof the second transistor electrically; an output terminal of the powersource voltage controller is connected to a second electrode of thefirst light emitting element and the second electrode of the secondlight emitting element electrically; and a voltage between the sourceand drain of the second transistor is controlled to set the voltagebetween the source and drain of the first transistor in the vicinity ofthe boundary between the saturation region and the linear region.
 29. Acontrol method of a display-device according to claim 24: wherein thepower source voltage controller is an operational amplifier.
 30. Acontrol method of a display device according to claim 25: wherein thepower source voltage controller is an operational amplifier.
 31. Acontrol method of a display device according to claim 26: wherein thepower source voltage controller is an operational amplifier.
 32. Acontrol method of a display device according to claim 28: wherein thepower source voltage controller is an operational amplifier.
 33. Adisplay device according to claim 1: wherein the power source voltagecontroller is a switching regulator.
 34. A display device according toclaim 2: wherein the power source voltage controller is a switchingregulator.
 35. A display device according to claim 3: wherein the powersource voltage controller is a switching regulator.
 36. A display deviceaccording to claim 4: wherein the power source voltage controller is aswitching regulator.
 37. A display device according to claim 11: whereinthe power source voltage controller is a switching regulator.
 38. Adisplay device according to claim 12: wherein the power source voltagecontroller is a switching regulator.
 39. A display device according toclaim 13: wherein the power source voltage controller is a switchingregulator.
 40. A display device according to claim 14: wherein the powersource voltage controller is a switching regulator.
 41. A control methodof a display device according to claim 24: wherein the power sourcevoltage controller is a switching regulator.
 42. A control method of adisplay device according to claim 25: wherein the power source voltagecontroller is a switching regulator.
 43. A control method of a displaydevice according to claim 26: wherein the power source voltagecontroller is a switching regulator.
 44. A control method of a displaydevice according to claim 28: wherein the power source voltagecontroller is a switching regulator.